As illustrated schematically in FIG. 1, an active matrix of an AMOLED display screen usually comprises n×m pixels pixi,j, where i=1 to m and j=1 to n, m row select lines Scan i and n columns DATA j allowing n pixels of the display screen to be addressed. For example, the pixel pixi,j may be controlled so as to display video information by means of the column DATA j, via which a voltage corresponding to the video information to be displayed is applied, and a row select line Scan i.
The invention concerns particular current control structures for a pixel which make it possible to solve the known problem of display degradation in AMOLED display screens due to the drift in threshold voltage of the current control transistors that drive the organic light-emitting diodes.
These structures make it possible to provide a phase for recovering the drift in threshold voltage of the current control transistors for each pixel by means of a structure having two current drivers. More precisely, as shown in FIG. 2, each pixel pixi,j comprises a structure having two drivers COM and COM′ for driving the organic diode OLED of the pixel. These two drivers COM and COM′ each have an output that controls an electrode E1 of the OLED diode, which corresponds to the pixel electrode produced on the active matrix, whereas the other electrode E2 (cathode) of the diode, which corresponds to the counter-electrode, is connected to a potential Vk common to all the pixels.
The drivers COM and COM′ have an identical structure, comprising at least: a switching transistor, a current control transistor and a sustain capacitor. By convention, T1, C1 and T2 denote these elements of the first referenced driver COM, and T1′, C1′ and T2′ these elements of the second referenced driver COM′.
These drivers COM and COM′ are controlled by row select lines and/or column data lines of the matrix in such a way that they have a different function at each video frame, these functions being periodically reversed. These functions are: 1) to display video information, by application of a video voltage to the gate of the current control transistor, so as to deliver a corresponding current to the OLED diode; and 2) to compensate for the stress occasioned by the first function, by application of a blocking voltage to the gate of the current control transistor.
Thus, during a given video frame, one of the two drivers, for example the driver COM, has the function of displaying the video information via the OLED diode, by application of a corresponding video voltage on the gate of its current control transistor T2, while the other driver, in the example COM′, has the function of applying a blocking voltage to the gate of its current control transistor T2′, thereby placing this transistor in threshold voltage drift recovery phase.
The functions of the two drivers are periodically reversed, so that one frame in two on average is used, for each driver, in order to destress its current control transistor. The average threshold voltage drift of each current control transistor of the active matrix is zero or approximately zero. This may be obtained without affecting the duty cycle for applying the video voltage to the OLED diode, so that the diode remains continually controlled (100% duty cycle).
To control the two drivers COM and COM′ appropriately, as indicated above, rows and/or columns of the matrix are used, with, where appropriate, additional rows or columns especially provided, depending on the structure of the drivers. More precisely, a first embodiment has a control structure based on four transistors, two per driver COM, COM′ (namely a switching transistor T1, T1′ and a current control transistor T2, T2′) and additional rows or columns are inserted into the matrix in order to drive them appropriately. Another embodiment has a control structure based on six transistors, three per driver COM, COM′ (namely two switching transistors and one current control transistor) and row select lines for the preceding and following pixels of the matrix are used in addition to the usual row select line and column line for driving the transistors of the drivers COM and COM′ appropriately.
One technical problem that arises in the invention is how to increase the number of transistors per pixel and/or the number of rows or columns needed to control each pixel, while still obtaining an open aperture ratio for a given pixel size (and therefore active matrix size) and an optimum manufacturing yield.